This invention relates in general to inkjet printers and in particular to methods of speeding the process of positioning a printer""s drive roller for sequential printhead swaths.
Many matrix-type printers, including inkjet printers, operate by repeatedly sweeping a printhead transversely over a print medium to print a number of dot rows. Each such sweep is referred to as a swath. The print medium is advanced longitudinally between each swath, so that the entire surface of the print medium is eventually covered.
As printing technologies improve and the pitch of the dot rows decreases, the accuracy with which the print medium advances becomes more critical. Inaccuracies in print medium positioning result in artifacts or bands on the printed page. For example, inaccuracies might cause two swaths to partially overlap, creating a noticeable and undesirable band of dot rows that has been printed twice.
Current media advance systems often use closed-loop servos to improve accuracy. However, increasing requirements for accuracy often limit the speed at which such servo systems can operate. Typically, significant inaccuracies and control system instabilities can be traced to drive train backlash. Accordingly, an effort is made to avoid overshoot when positioning the print medium, thereby avoiding backlash effects. To accomplish this, two positioning steps are often used. In a first step, the system slews at a relatively fast speed to an initial target position that is well short of the final desired position. The initial target position is selected so that the positioning step always stops short of the ultimate desired position, accounting for the worst case of positioning overshoot. In a second step, the servo system xe2x80x9ccreepsxe2x80x9d forward very slowly to the final target. The slow speed avoids overshooting during this second positioning step, resulting in very accurate positioning.
During the slew portion of the positioning procedure, there is a variable amount of positioning error. This error, as an example, might vary between plus and minus four dot rows. To avoid overshoot, the initial target position is selected to be four dot rows short of the final target, thereby ensuring that overshoot will not occur. Given, however, that both positive and negative positioning errors might occur, the actual position attained during the slew positioning step might be as much as eight dot rows short of the final target position. Thus, the distance from the initial target position to the final desired position can be quite large. As a result, the second positioning step often accounts for a significant portion of the total media advance time, and throughput is severely restricted by the xe2x80x9ccreepingxe2x80x9d of the print medium toward its final position.
Some printers provide a way to improve speed by implementing a xe2x80x9cdraft mode,xe2x80x9d in which the second positioning step is simply omitted. However, the quality of printing is noticeably poorer in this mode.
A printer in accordance with the invention uses a printhead that has more nozzle rows than the number of dot rows in any given swath. The printer uses a variable subset of adjacent nozzle rows to account for drive roller positioning inaccuracies. For example, in one embodiment of the invention the printer moves its drive roller in a single advance using a relatively fast (but inaccurate) slew speed. Any resulting positioning inaccuracies are accounted for by ascertaining the actual position of the drive roller after the single advance and by then selecting a subset of nozzles that is closest in position to the actual positions of the desired dot rows after the single advance procedure. As an improvement to this method, a second advance can be utilized, at a slow xe2x80x9ccreepxe2x80x9d speed, to advance the print medium so that the dot rows of the current swath lie precisely (within the tolerance of the position feedback mechanism) beneath the selected subset of nozzles. Because of selecting the closest set of nozzles, the distance of this second move is never more than a single dot row.
The invention can also be used in a system that uses a stepper motor instead of a servo-feedback system, to achieve dot pitches that are smaller than the positioning resolution of the stepper motor. In a system such as this, the stepper motor is advanced as closely as possible to the target position, given the limited resolution of the stepping mechanism. Then, a subset of nozzles is selected to correspond as closely as possible to the desired positions of the dot rows. Because of the finer pitch of the dot rows on the printhead, this results in dot row placement with a resolution that exceeds that of the stepper motor.